An ion source is a device that ionizes gas molecules and then focuses, accelerates, and emits them as a narrow beam. This beam is then used for various technical and technological purposes such as cleaning, activation, polishing, thin-film coating, or etching.
For better understanding the principle of the present invention, it would be expedient to describe in detail a known ion-beam source of the type to which the invention pertains. Such an ion source is described, e.g., in Russian Patent No. 2,030,807 issued in 1995 to M. Parfenyonok, et al. The patent describes an ion source that comprises a magnetoconductive housing used as a cathode having an ion-emitting slit, an anode arranged in the housing symmetrically with respect to the emitting slit, a magnetomotance source, a working gas supply system, and a source of electric power supply.
FIGS. 1 and 2 schematically illustrate the aforementioned known ion source with a circular ion-beam emitting slit. More specifically, FIG. 1 is a sectional side view of an ion-beam source with a circular ion-beam emitting slit, and FIG. 2 is a sectional plan view along line II--II of FIG. 1.
The ion source of FIGS. 1 and 2 has a hollow cylindrical housing 40 made of a magnetoconductive material such as Armco steel (a type of a mild steel), which is used as a cathode. Cathode 40 has a cylindrical side wall 42, a closed flat bottom 44 and a flat top side 46 with a circular ion emitting slit 52. A working gas supply hole 53 is formed in flat bottom 44. Flat top side 46 functions as an ion-accelerating electrode. Placed inside the interior of hollow cylindrical housing 40 between bottom 44 and top side 46 is a magnetic system in the form of a cylindrical permanent magnet 66 with poles N and S of opposite polarity. An N-pole faces flat top side 46 and S-pole faces bottom side 44 of the ion source. The purpose of a magnetic system 66 with a closed magnetic circuit formed by parts 66, 46, 42, and 44 is to induce a magnetic field in ion emitting slit 52. It is understood that this magnetic system is shown only as an example and that it can be formed in a manner described, e.g., in U.S. Pat. No. 4,710,283 issued to Singh, et al. in 1987. A circular annular-shaped anode 54 which is connected to a positive pole 56a of an electric power source 56 is arranged in the interior of housing 40 around magnet 66 and concentric thereto. Anode 54 is fixed inside housing 40 by means of a ring 48 made of a non-magnetic dielectric material such as ceramic. Anode 54 has a central opening 55 in which aforementioned permanent magnet 66 is installed with a gap between the outer surface of the magnet and the inner wall of opening 55. A negative pole 56b of electric power source is connected to housing 40 which is grounded at GR.
Located above housing 40 of the ion source of FIGS. 1 and 2 is a sealed vacuum chamber 57 which has an evacuation port 59 connected to a source of vacuum (not shown). An object OB to be treated is supported within chamber 57 above ion emitting slit 52 by an insulator block 61 rigidly attached to the housing of vacuum chamber 57 by a bolt 63 but so that object OB remains electrically isolated from the housing of vacuum chamber 57. However, object OB is electrically connected via a line 56c to negative pole 56b of power source 56. Since the interior of housing 40 communicates with the interior of vacuum chamber 57, all lines that electrically connect power source 56 with anode 54 and object OB should pass into the interior of housing 40 and vacuum chamber 57 via conventional commercially-produced electrical feedthrough devices which allow electrical connections with parts and mechanisms of sealed chambers without violation of their sealing conditions. In FIG. 1, these feedthrough devices are shown schematically and designated by reference numerals 40a and 57a. Reference numeral 57b designates a seal for sealing connection of vacuum chamber 57 to housing 40.
The known ion source of the type shown in FIGS. 1 and 2 is intended for the formation of a unilaterally directed tubular ion beam. The source of FIGS. 1 and 2 forms a tubular ion beam IB emitted in the direction of arrow A and operates as follows.
Vacuum chamber 57 is evacuated, and a working gas is fed into the interior of housing 40 of the ion source. A magnetic field is generated by magnet 66 in the accelerating gap between anode 54 and cathode 40, whereby electrons begin to drift in a closed path within the crossed electrical and magnetic fields. A plasma 58 is formed between anode 54 and top cathode plate 46. When the working gas is passed through an ion-acceleration and ionization gap 52a (hereinafter referred to as "ionization gap"), tubular ion beam IB, which is propagated in the axial direction of the ion source shown by an arrow A, is formed in the area of an ion-emitting slit 52 and in ionization gap 52a between anode 54 and top cathode plate 46.
The above description of the operation of the ion source is simplified to ease understanding of the principle of the invention. In reality, the phenomenon of generation of ions in the ion source with a closed-loop drift of electrons in crossed electric and magnetic fields is of a more complicated nature and consists in the following.
When, at starting the ion source, a voltage between anode 54 and cathode 40 reaches a predetermined level, a gas discharge occurs in gap 52a. As a result, the electrons, which have been generated as a result of ionization, begin to migrate towards anode 54 under the effect of the electric field, colliding with the molecules of working gas and moving along specific trajectories described below. The space in which the electrons drift is confined between an inner part 46a and an outer part 46b of top cathode plate 46, which form ion-emitting slit 52, and the surface of anode 54 facing top cathode plate 46.
The principle of operation of the ion-beam source to which the present invention pertains can be better understood after consideration of a direct current vacuum magnetron a part of which is shown schematically on FIG. 1A. If one assume that in ion source of FIG. 1 ion-emitting slit 52 is absent and that the magnetic field B between cathode 46' and anode 54' passes parallel to the planes of the anode and cathode (i.e., perpendicular to the plane of the drawing), then such a system can be considered as the aforementioned direct current vacuum magnetron (hereinafter referred to as "DC magnetron").
In a DC magnetron, the electrons, which are emitted from cathode 46', move toward anode 54'. However, their trajectory is curved under the effect of magnetic field B. When the strength of magnetic field B exceeds a predetermined critical value B.sub.cr, the electrons do not reach the surface of anode 54' and return back to cathode 46'. More specifically, the electrons begin to move along cycloidal trajectories shown in FIG. 1A. As a result, the electrons are accumulated in the space between cathode 46' and anode 54', and their concentration can reach a significant value. It is known that height H of such a cycloid is equal to so-called doubled Larmor radius R.sub.L which is represented by the following formula: EQU R.sub.L =m.sub.e V/.vertline.e.vertline.B,
where m.sub.e is a mass of the electron, B is the strength of the magnetic field inside the slit, V is a velocity of the electrons in the direction perpendicular to the direction of the magnetic field, and .vertline.e.vertline. is the charge of the electron (see D. L. Smith. "Thin-Film Deposition". Principles and Practice. McGraw-Hill Inc., New York, p. 384, 1995).
Based on the principle described above, in the construction of ion-beam source shown in FIG. 1, the distance between anode 54 and cathode 46 should be equal to or greater than two Lamnor radii of electrons in the magnetic field.
In contrast to D.C. magnetron shown in FIG. 1A, real ion-beam source of FIG. 1 has a closed-loop ion emitting slit 52 required for forming, extracting, and emitting an ion beam IB toward an object OB. The presence of ion-emitting slit 52 leads to non-uniformities in electric and magnetic fields in the area above anode 54, i.e., in gap 52a and in ion-emitting slit 52. This makes the electron drift pattern more complicated than shown in FIG. 1A. The electrons begin to drift not only in gap 52a, but also in ion-emitting slit 52. These drifting electrons are responsible for the following two processes: 1) they collide with molecules of the working gas, ionizes them, and thus form positive ions; 2) they compensate for the positive spatial charge of the ion beam.
It should be noted that strictly speaking electrons do not drift in a plane in the ion-emitting slit. However, for the convenience of the description, here and hereinafter such as expressions as "plane of drift of electrons", "drift in the direction of propagation of the ion beam", etc., will be used.
In ion source of FIG. 1, the magnetic field is localized essentially between top parts 46a and 46b of top cathode plate 46, i.e., in ion-emitting slit 52 and near this slit. This magnetic field practically does not influence on the trajectories of the ions. This is because the Larmor radius of the ion is (m.sub.i /m.sub.e).sup.1/2 times the Larmor radius of the electron, where m.sub.I is mass of ion and m.sub.e is mass of electron. For example, for an ion of argon having m.sub.i =40 atomic units, the Larmor radius of the ion is 270 times the Larmor radius of the electron.
When a working medium, such as argon which has neutral molecules, is injected into the ionization space inside housing 40, the molecules are ionized by the electrons present in this space and are accelerated by the electric field. As a result, ions are formed and emitted from the slit towards the object.
In the space above anode 56 and in ion-emitting slit 52, the electrons are maintained in high concentration under the effect of crossed electric and magnetic fields. This high concentration ensures effective ionization of the working gas and compensates for the abovementioned positive spatial charge. Thus, it becomes possible to form high-intensity ion beams from various gaseous substances.
The diameter of the tubular ion beam formed by means of such an ion source may reach 500 mm and more.
The ion source of the type shown in FIG. 1 is not limited to a cylindrical configuration and may have an elliptical or an oval-shaped cross section as shown in FIG. 3. FIG. 3 is a cross-sectional view of the ion-beam source having an oval cross section. In FIG. 3 the parts of the ion beam source that correspond to similar parts of the previous embodiment are designated by the same reference numerals with an addition of subscript OV. Structurally, this ion source is the same as the one shown in FIG. 1 with the exception that a cathode 40.sub.ov, anode 54.sub.ov, a magnet 66.sub.ov, and hence an emitting slit (not shown in FIG. 3), have an oval-shaped configuration. As a result, a belt-like ion beam having a width of up to 1400 mm and more can be formed. Such an ion beam source is suitable for treating large-surface objects when these objects are passed over ion beam IB emitted through emitting slit 52.
With 1 to 3 kV voltage on the anode and various working gases, this source makes it possible to obtain ion beams with currents of 0.5 to 1 A. In this case, an average ion energy is within 400 to 1500 eV, and nonuniformity of treatment over the entire width of a 1400 mm-wide object does not exceed .+-.5%.
However, in the ion sources of the type shown in FIGS. 1 through 3, the volume of the space where electrons drift is limited by metallic anode 54, which should be located close to ion-emitting slit 52. This decreases the residence time of the electrons in a free state and thus decreases efficiency of ionization of the working gas. When the working medium comprises a polyatomic gas, such as SF.sub.6, negatively charged ions or high-velocity neutral particles may appear in the near-anode area. These ions and particles may lead to erosion of the anode, and thus to contaminate the ion beam of the source with the material of the anode.